Gaseous fueled vehicles may store fuel in pressurized tanks (made from carbon fiber, for example) at maximum pressures of approximately 350 to 700 bar. Significant energy may be stored in the compressed gas at these high pressures and such energy may be lost when the pressure is throttled down to the 5-10 bar range of port fuel injection type engines, or to the 50-150 bar range of direct injection engine.
One approach aimed at recovering at least some of the compressed gas energy is described in U.S. Pat. No. 5,941,210. The disclosed approach uses a turbocharger run by the compressed gas flow to recover expansion energy during the regulation process when the gaseous fuel pressure is reduced to supply fuel to a direct injection engine. It also describes injection at variable fuel pressures without regulation, where a control means responds to changes in the pressure of the fuel supply system. In this regard, a pressure measurement device is provided in the fuel supply system.
However, the inventors herein have recognized that in some cases, such a system may increase the cost and/or complexity of the fuel supply system because of the addition of the turbocharger and associated equipment, which can further complicate operation in the event of degradation in the turbocharger and/or associated equipment. This can decrease the ability to accurately regulate fuel pressure and/or recover compressed gas energy. Further, when relying on pressure measurements to respond to changes in fuel pressure, oscillations in fuel pressure caused by actuation of the injectors can generate errors in fuel delivery, again decreasing the ability to accurately regulate fuel pressure. Such degraded operation may be especially prevalent under widely varying tank pressures experienced when trying to recover compressed gas energy in the fuel system.
To address at least some of the above issues, various methods may be provided. In one embodiment, a method may include: directly injecting fuel from the injector into the chamber at a variable supply pressure which decreases as fuel tank pressure decreases; and adjusting at least one of an injection timing and duration in response to at least said variable pressure and information from the exhaust oxygen sensor. In one example, the injector may be a piezoelectric type injector with sufficiently fast response time to enable compensation for widely varying fuel pressure, thereby reducing pressure loss due to regulation. Further, the timing of the injection may be later in the compression stroke and in the expansion stroke (during closed intake and exhaust valve conditions) to enable recovery of the compressed gas energy. Of course, additional injections during the cycle, and other fast-response injector designs, may also be used.
The above approach may have various advantages. For example, by adjusting injection control in response to both the variable fuel pressure and feedback from an exhaust gas oxygen sensor, it may be possible to accurately control gaseous fuel delivery over a widely varying fuel pressure range, even in the face of widely varying fuel delivery amounts and timing across the engine speed and load range. In this way, compressed energy in the stored fuel may be recovered in the cylinder while still maintaining acceptable fuel control.
Note that the above approach may be used without pressure regulation, although it is also applicable, if not more applicable, to systems including at least some pressure regulation in the fuel system, which may include variable pressure regulation. For example, under engine operating conditions of reduced injector pulsewidth, increased pressure regulation may be used, whereas during conditions of increased injector pulsewidth, reduced pressure regulation may be used, thereby enabling accurate control and increased compressed gas energy recovery when possible.